1. Field of the Invention
The present invention relates to a NAND-type magnetoresistive RAM, and more specifically, to an improved cell structure of a magnetoresistive RAM, thereby reducing an effective area per cell and improving integration of a device.
2. Description of the Related Art
Recently, semiconductor memory manufacturers have developed a magnetoresistive RAM using ferromagnetic materials as a next generation memory device.
The magnetoresistive RAM having a multi-layered structure of ferromagnetic thin films is a memory device which can read and write data by detecting current variations depending on direction of magnetization of each thin film. The magnetoresistive RAM has a high operating speed, large capacitance and low power consumption due to characteristics of magnetic thin film. A nonvolatile memory operation performed on a flash memory can be performed on the magnetoresistive RAM.
The study on the magnetoresistive RAM is in the initial stage, and focused on formation of multi-layered magnetic thin films. The studies on unit cell structures or adjacent detection circuit are still on the early stage FIG. 1 illustrates a cross-sectional diagram of a MTJ cell (Magnetic Tunnel Junction cell), having a multi-layered magnetic thin film structure, of a conventional magnetoresistive RAM.
In general, a MTJ cell 5 is formed of an anti-ferroelectric layer 1, a fixed ferromagnetic layer 2, a thin insulating layer 3 where tunneling current flows and a free ferromagnetic layer 4.
Here, the fixed ferromagnetic layer 2 has an fixed direction of magnetization. The anti-ferroelectric layer 1 fixes the direction of magnetization of the fixed ferromagnetic layer 2. However, direction of magnetization of the free ferromagnetic layer 4 are changed by an external magnetic field. The free ferromagnetic layer 4 can store data “0” or “1” depending on its direction of magnetization .
When current flows vertically in the MTJ cell 5, tunneling current is generated through the thin insulating layer 3. Here, if the direction of magnetization of the fixed ferromagnetic layer 2 is the same with that of the free ferromagnetic layer 4, the large amount of the tunneling current flows. However, if the direction of magnetization of the fixed ferromagnetic layer 2 is opposite to that of the free ferromagnetic layer 4, the small amount of the tunneling current flows.
This phenomenon is called a TMR (Tunneling Magnetoresistance) effect. By detecting the amount of the tunneling current, the direction of magnetization of the free ferromagnetic layer 4 can be found out, and data stored in a cell can be read.
FIG. 2a illustrates a unit cell of the conventional magnetoresistive RAM using a field effect transistor.
The unit cell of the magnetoresistive RAM comprises a the MTJ cell 5, a read wordline 6, a bitline 7, a write wordline 8 and a MOS(metal-oxide-silicon) field effect transistor 9.
Here, the read wordline 6 is used when data is read. The write wordline 8 forms an external magnetic field according to application of current. As a result, data can be stored depending on variations of direction of magnetization of the free ferromagnetic layer 4 in the MTJ cell 5. The bitline 7 applies current vertically to the MTJ cell 5. As a result, the direction of magnetization of the free ferromagnetic layer 4 can be found out.
In the conventional magnetoresistive RAM, the MOS field effect transistor 9 is operated by applying a voltage to the read wordline 6 in a read mode. Then, the amount of current flowing in the MTJ cell 5 is detected after current is applied to the bitline 7.
In a write mode, the MOS field effect transistor 9 is maintained at an off state, and the current is applied to the wordline 8 and the bitline 7. A resultant external magnetic field changes the direction of magnetization of the free ferromagnetic layer 4 in the MTJ cell 5.
Here, the current is simultaneously applied to the bitline 7 and write wordline 8 because the largest magnetic field is generated at a point where the two metal lines are vertically crossed. As a result, one cell is selected among a plurality of cells.
FIG. 2b illustrates a cross-sectional diagram of the unit cell of the conventional magnetoresistive RAM.
Referring to FIG. 2b, a ground line 12 is formed on a source 10 of the vertical MOS field effect transistor 9. The read wordline 6 is formed on a gate of the MOS field effect transistor 9. A conductive layer 13, a contact plug 14, a conductive layer 15 and a contact plug 16 are sequentially formed on a drain 11 of the MOS field effect transistor 9. A connection layer 17 is formed on the write wordline 8. The MTJ cell 5 and the bitline 7 are formed on the connection layer 17 as a stacked structure.
A cell of the conventional magnetoresistive RAM comprises a transistor, a MTJ cell, a read wordline, a write wordline and a bitline. As a result, an effective area of the cell is increased, and the integration of the memory device is degraded.